The present invention relates to an improved sprue bar assembly having a mechanism for automatically adjusting the length of the sprue bar in a stack mold, or stack mold carrier assembly, to accommodate molds of different shutheight. The improved sprue bar assembly of the present invention can also be used to facilitate injection compression and/or coining processes in stack molds.
Stack molds typically rely upon hot runner systems for conveying melted plastic to their mold cavities. Generally, a hot runner manifold is mounted in the moving center section of the stack mold or is fastened to a movable stack mold carrier that may be part of the machine. A sprue bar extends from the hot runner manifold and seals to the machine injection nozzle. This sealed connection is typically broken and remade for each molding cycle. When the mold must be opened to eject the molded parts, the pressurized hot runner manifold and sprue bar melt channels must be relieved of pressure in order to prevent drool or leakage of the plastic when the melt channel sections must be separated. However, this pressure relief must not allow air to be introduced into the melt stream when the mold is closed for the next injection cycle.
A stack mold carrier is sometimes installed in a machine to allow a variety of different stack molds to be installed or removed quickly. The carrier typically has a linkage system for opening and closing the various mold sections and sometimes includes the hot runner and sprue bar so that only the mold cavities and cores need be changed. If a mold core and cavity set is installed that has a different shutheight from the set it is replacing, a different sprue bar of suitable length must also be installed to match the new shutheight. This is both expensive and time consuming.
Several techniques have been used to solve the problem of stack mold hot runner depressurization. Swiss patent 625,461 to Hotz illustrates a stack mold hot runner manifold having a retractable pin in the melt channel which when retracted increases the volume of the melt channel, thereby depressurizing it prior to the mold opening. After closing for the next cycle, the pin must be advanced in order to be ready for the next depressurizing action. Japanese Patent Document No. 62-35817 illustrates a similar depressurizing pin.
French Patent No. 2,152,932 illustrates a spring loaded bushing on an offset sprue bar that can slide away from the hot runner nozzle, thereby increasing the volume of the melt channel to allow depressurization as the mold opens. The mold closing action recompresses the springs for the next cycle. Further examples of stack mold applications are shown in U.S. Pat. Nos. 5,458,843 and 5,464,579, both to Brown. Also, FIG. 29 in an article xe2x80x9cProgress of Automatic Moldsxe2x80x9d from Japan Plastics Age shows a spring loaded bushing used in a stack mold application.
Other manifold depressurization mechanisms are illustrated in U.S. Pat. No. 4,299,791 to Aoki, U.S. Pat. No. 3,934,626 to Hall and Japanese Patent Document No. 51-102047. Each of these documents shows spring loaded bushings. U.S. Pat. No. 4,473,347 to Terashima illustrates a pair of opposed sliding blocks that create additional melt channel volume in the manifold to effectively depressurize the manifold.
Allowed co-pending U.S. patent application Ser. No. 09/054,692 to Teng et al., now U.S. Pat. No. 6,027,681 which is hereby incorporated by reference herein, illustrates a stack mold carrier linkage system having a linkage that can accommodate molds of different shutheights. There is no teaching in this application however of adjusting the sprue bar length to suit the differing mold shutheights.
U.S. Pat. No. 5,910,327 to Schad illustrates a telescoping sprue bar that allows a shorter sprue bar to be used in a stack mold. However, the Schad design does not facilitate adjustment to accommodate differing mold shutheights. Japanese Patent Document No. 6-000848 also illustrates a telescoping sprue bar.
U.S. Pat. No. 3,843,295 to Greenberg illustrates an adjustable injection nozzle in a stack mold application whereby the nozzle can be lengthened by using springs or fluid pressure. In this patent, multiple nozzles protrude through the center section of the stack mold to supply resin to the mold cavity sets distant from the machine""s injection unit. This impractical mold layout does not work in situations where the molds are to be changed in a manner that minimizes time and cost. Also, the nozzle extensions that pass through the mold set adjacent the machine""s injection unit in order to reach the distant mold set are impediments when the mold sets open and must eject their molded parts.
U.S. Pat. No. 4,212,626 to Gellert illustrates two opposed hot runner valve gated nozzles that are used to control the melt flow across the parting line of a stack mold, thereby preventing leakage and drool when the mold opens. Decompressing the manifold is not discussed and such a design would not be easily adjustable for molds of differing shutheights.
Co-pending U.S. patent application Ser. No. 09/173,783 now U.S. Pat. No. 6,135,757 illustrates the concept of providing grooves on the outer surface of valve gate stem to facilitate closing the stem against compressed melt and reducing the heat loss from the stem. Providing a channel within the stem to provide a venting route for excess melt is not contemplated.
Injection compression molding and coining are processes whereby melted resin is fed into a mold that is not completely closed and clamped such that after feeding a metered amount of resin into the mold, the supply is shut off and the mold closed and clamped compressing the melt to fill the cavity entirely. This process provides the molded part with enhanced properties, typically less residual stress and sometimes allows parts of thinner cross section to be molded. U.S. Pat. No. 5,130,075 to Hara illustrates such a process in which resin is fed into a mold that is allowed to open during filling and then is closed and clamped to complete the process. Processes like injection compression and coining have not previously been possible to perform in stack mold applications because the movement of the mold during the filling and/or compressing stage could not be facilitated with a fixed length sprue bar.
Accordingly, it is an object of the present invention to provide a sprue bar assembly that accommodates molds of different shutheights.
It is a further object of the present invention to provide a sprue bar assembly as above that decompresses the melt channel.
It is yet a further object of the present invention to provide improved compression and/or coining processes for making molded articles.
The foregoing objects are attained by the sprue bar assembly of the present invention.
In accordance with the present invention, a sprue bar assembly is provided for use with an injection molding machine having at least one mold and for accommodating different mold shutheights. The sprue bar assembly comprises a fixed sprue bar section having a first channel through which molten material, such as a plastic resin, flows. The first channel communicates with the outlet of a source of the molten material. The sprue bar assembly further comprises means for accommodating different mold shutheights. The accommodating means comprises an adjustable sprue bar section having a second channel through which molten material flows and powered means for holding the adjustable sprue bar section in contact with the fixed sprue bar section so that molten material can flow from the first channel to the second channel without leaking. In a preferred embodiment of the present invention, the fixed sprue bar section and the adjustable sprue bar section are each provided with a valve gate for preventing drooling or leakage of the molten material when the two sections are separated.
The adjustable sprue bar assembly of the present invention can be used in a number of different molding machines for performing a plurality of different molding processes. For example, the adjustable sprue bar assembly of the present invention can be used to facilitate injection compression and/or coining processes in stack molds.
Other details of the sprue bar assembly of the present invention and its applications, as well as other objects and advantages attendant thereto, are set forth in the following description and the accompanying drawings in which like reference numerals depict like elements. dr
FIG. 1 is a side view of a stack mold using a stack mold carrier and having a sprue bar assembly in accordance with the present invention;
FIG. 2 is a side view in partial cross section of the sprue bar assembly in accordance with the present invention while a mold is in a closed position;
FIG. 3 is a side view in partial cross section of the sprue bar assembly in accordance with the present invention while a mold having an increased shutheight is in a closed position;
FIG. 4 is a side view in partial cross section of the sprue bar assembly in accordance with the present invention while a mold is in a partially mold open position;
FIG. 5 is a side view in partial cross section of the sprue bar assembly in accordance with the present invention while a mold is in a final mold open position;
FIG. 6 is a top view of a hot runner assembly including a sprue bar assembly in accordance with the present invention;
FIG. 7A is a side view of an alternate sprue bar nozzle assembly with a valve gate in an open position;
FIG. 7B is a side view of the alternate sprue bar nozzle assembly of FIG. 7A with the valve gate in a closed position;
FIG. 7C is a sectional view of a tip of a valve stem used in the valve gate shown in FIG. 7A;
FIG. 8 is a side view in partial cross section of a sprue bar assembly in accordance with the present invention used in an injection-compression molding application in a mold filling stage;
FIG. 9 is a side view in partial cross section of the sprue bar assembly of FIG. 8 in a compressing/coining stage;
FIG. 10 is a side view in partial cross section of the sprue bar assembly of FIG. 8 in a partially mold open position;
FIG. 11 is a side view in partial cross section of the sprue bar assembly of FIG. 8 in a final mold open position; and
FIG. 12 is a side view in partial cross section of an alternative sprue bar assembly in accordance with the present invention.